1. Field of the Invention
The present invention relates to a start-up apparatus for a power converter. More particularly, the present invention relates to a start-up apparatus having a hysteresis threshold voltage range to control the power converter.
2. Description of Related Art
FIG. 1 shows a power converter comprising a switching controller U1. The switching controller U1 is utilized to control the power converter for regulating a stable output. However, a supply voltage is needed for powering an internal control circuit. Further referring to FIG. 1, as an input voltage VIN is applied to the power converter, the input voltage VIN will charge up a holding capacitor CST via a start-up resistor RST. The switching controller U1 will then start to switch a transistor T1 via a power switch Q1. After the power supply is started up, an auxiliary winding NA of the transformer T1 further powers the switching controller U1 via a diode DA.
The start-up resistor RST works only for starting up the power converter. After the power converter is started up, the start-up resistor RST only consumes unnecessary power and becomes redundant. A power consumption PR of the start-up resistor RST can be expressed by equation (1).
                              P          R                =                                            (                                                V                  IN                                -                                  V                  DD                                            )                        2                                R            ST                                              (        1        )            
Power-related organizations in the world have set up a series of specification for green-mode power saving. Therefore, it is more desirable for power converter designers to make the start-up resistor RST open circuit or to provide a solution without the start-up resistor for saving power, especially under no conditions.